1. Field of the Invention
This invention generally relates to wireless telephone communications and, more particularly, to a system and method for using a common filter for Time Division Multiple Access (TDMA) transmit and receive communications.
2. Description of the Related Art
FIG. 5 is a schematic block diagram depicting an automatic switch, a transmit bandpass filter, and a receive bandpass filter in a wireless communications device transceiving half duplex communications (prior art). As shown in FIG. 5, wireless devices transceiving half duplex communications typically have a fixed-tuned Tx bandpass filter (BPF) and a fixed-tuned Rx BPF to meet filtering specifications. Current system architecture forces the Tx BPF and the Rx BPF to each have a bandwidth sufficient to accommodate operation of any Tx or Rx single channel in any region of the respective Tx or Rx system band.
These fixed-tuned filters have the contradictory objectives of achieving the lowest possible passband insertion loss (IL) while simultaneously achieving a specified large out-of-band rejection and small size. Selectivity over the full range of the Tx and Rx system passbands is obtained using relatively complex Tx and Rx filters. That is, the order of the filters (number of resonators), is relatively large. High order (greater than 2nd order) fixed-tuned filters constructed from either individual coaxial resonator elements or monoblock structures are conventionally used. Complex Tx and Rx BPFs negatively impact a wireless device. First, using a higher order filter quickly increases the IL of the filter. That is, as the number of resonators in the filters increases, the filters become more lossy. In addition, to satisfy out-of-band rejection specifications, a transmission zero is usually required, with the added disadvantage of increasing IL at the band edge. Second, increasing the number of resonators in the filters typically increases the costs for manufacturing the filters. Because of variations in ceramics and fabrication tolerances, vendors must individually adjust the characteristics of fixed-tuned filters during their manufacture, further increasing costs. Third, more complex filters require more space in a wireless device. Regarding the last point, the desire to make smaller devices with increased functionality creates a need to reduce the number or size or both of the components in devices. However, increasing the number or size of filters can limit the size to which a wireless device can be reduced, or can limit space available in the wireless device for other components.
Fixed-tuned BPFs also can act to limit the useable applications for the wireless device containing the BPFs. For example, PCS bands in different geographical areas such as the U.S., Korea, and India have different frequency band specifications. Therefore, if more than one PCS frequency band is to be supported in a wireless device (for example, if the wireless device is to be useable in more than one of the above countries), multiple fixed-tuned BPFs are necessary, further exacerbating the disadvantages noted above. Such multiple BPFs would be necessary even if the power amplifier and low noise amplifier used in the wireless device have sufficient bandwidth to operate over these multiple bands.
It would be advantageous if the width of filter passbands in a wireless device transceiving half duplex communications could be reduced.